Electric resistance welding heads with electrodes located on the same side are known, in particular of the type comprising:                a support structure, provided with a connecting flange for connection to a wrist of an articulated robot,        an electric transformer, carried by the support structure, having two input poles for connection to electric supply cables of said head and having two output poles,        a ground electrode and a welding electrode carried by the support structure and electrically connected to the two output poles of the electric transformer,        wherein said ground electrode and said welding electrode have respective main axes parallel to and spaced from each other and said electrodes are movably mounted with respect to said support structure along the respective main axes,        an electrically operated actuator for driving a movement of said welding electrode with respect to said support structure along the respective main axis.        
A welding head having these characteristics is described for example in United States Patent Application Publication No. US2012/0067851 A1.
Welding heads of the type indicated above are for example used for welding motor-vehicle bodies, whereas conventional welding guns are not usable because one of the two electrodes of the welding gun is not adapted to reach an area to be welded and/or whereas it is preferred to avoid pushing of a welding electrode on a surface of the motor-vehicle body which then results evident in the final motor-vehicle.